Realization can be roughly understood as a kind of role-playing, a relationship between a property that plays a role and a property characterized by that role. This rough sketch previously received only moderate elaboration; recently, however, several substantive theories of realization have been proposed. But are there any general constraints on a theory of realization? What is a theory of realization supposed to accomplish? I first argue that a view of realization is viable, in part, to the extent that physical realization under that view explains or accounts for why instances of realized properties are necessitated by how things are physically in a modally strong sense. In this sense, I claim that physical realization should explain physical supervenience. I then call into question two alternative desiderata and raise a challenge for attempts to explicate realization in terms of isomorphism or analogy. Finally, I explain how a causal-functional account of realization, as well as less demanding accounts, can meet the preferred desideratum